A major interest of the x-ray crystallography working group, established last year within the Laboratory of Structural Biology, is broadly based in the study of the structure and function of macromolecular assemblies and multienzyme complexes. Other areas of interest within the group involve basic research on the structure of retroviral proteins and host cellular factors involved in HIV expression. The group is now near full strength and is well equipped with instrumentation and equipment. We have equipped ourselves with a modern x-ray diffraction data collection system, high-performance computer workstations, computer-graphics workstations, and facilities for preparative-scale expression and purification and crystallization of proteins. The group has spent a great deal of effort to obtain large amounts of pure proteins and macromolecular complexes and to obtain crystals suitable for high resolution x-ray diffraction studies. Protein expression studies of several cyclins are underway. We can now reliably produce large quantities of pure, soluble forms of human Nuclear Factor K-B (NFkB), the human histidyl TRNA synthetase, and retroviral intergrase. Several collaborators have demonstrated an ability to provide us with the quantities and purity of proteins needed for our structural studies. Small, but promising, crystals of whole LA viral particles, the "CLP-A" ATP-binding/protease regulatory subunits have been grown. Large, diffraction quality crystals of a bacterial phosphoribosyl transferase, and protease complex, human HLA Class 1/peptide complexes and the TRNA synthetase have been obtained. Full structure determinations are now underway. Three of these four structures will likely require us to use the more complicated and lengthy methods of multiple heavy-atom isomorphous replacement. All of these projects are complicated for a variety of technical reasons including the mechanical and radiation stability of the crystals or large unit cell parameters.